Acid excretion by bicarbonate-free perfused rat kidney.

We measured titratable acid (TA) and NH4 excretion by isolated rat kidneys perfused either with conventional bicarbonate-containing solutions or with solutions in which bicarbonate was replaced by propionate. Rates of TA excretion by bicarbonate-perfused kidneys were similar to in vivo values, 0.27 +/- 0.04 mueq.ml GF-1 (0.21 mueq.min-1.g-1), and increased significantly under bicarbonate-free conditions to 0.70 +/- 0.12 mueq.ml GF-1 (0.42 mueq.min-1.g-1). At the same time the perfusate/urine pH difference (delta pH) increased significantly, from 0.63 +/- 0.06 to 0.92 +/- 0.06. Carbonic anhydrase inhibition by 5 X 10(-4) M acetazolamide alkalinized the urine of bicarbonate-perfused kidneys, while in the bicarbonate-free preparation the urine remained acid (delta pH = 0.27 +/- 0.04) and titratable acid continued to be excreted, though at a reduced rate, 0.19 +/- 0.04 mueq.ml GF-1. Under these same bicarbonate-free carbonic anhydrase-inhibited conditions, lowering the perfusate pH from 7.4 to 7.1 increased delta pH to 0.36 +/- 0.02 and caused total acid excretion (TA + NH4) to rise from 0.29 +/- 0.04 to 0.45 +/- 0.06 mueq.ml GF-1, and increasing the perfusate [HPO4] from 2.4 to 9.6 mM increased TA to 0.80 +/- 0.09 mueq.ml GF-1.